linuxkit: allow user to configure a content trust passphrase command

Could be e.g.:
```
pkg:
  content-trust-passphrase-command: "lpass show <key> --password"
```
or
```
pkg:
  content-trust-passphrase-command: "gpg -d ~/.notary/passphrase.gpg"
```

Signed-off-by: Ian Campbell <ijc@docker.com>

docs/packages.md

@@ -94,6 +94,11 @@ invoke the build like this (for LastPass):
 ```
 DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE=$(lpass show <key> --password) linuxkit pkg push «path-to-package»
 ```
+or alternatively you may add the command to `~/.moby/linuxkit/config.yml` e.g.:
+```
+pkg:
+  content-trust-passphrase-command: "lpass show <key> --password"
+```
 
 ### Build packages as a developer
 

src/cmd/linuxkit/main.go

@@ -3,12 +3,27 @@ package main
 import (
 	"flag"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 
 	log "github.com/sirupsen/logrus"
+	"gopkg.in/yaml.v2"
 )
 
+// GlobalConfig is the global tool configuration
+type GlobalConfig struct {
+	Pkg PkgConfig `yaml:"pkg"`
+}
+
+// PkgConfig is the config specific to the `pkg` subcommand
+type PkgConfig struct {
+	// ContentTrustCommand is passed to `sh -c` and the stdout
+	// (including whitespace and \n) is set as the content trust
+	// passphrase. Can be used to execute a password manager.
+	ContentTrustCommand string `yaml:"content-trust-passphrase-command"`
+}
+
 var (
 	defaultLogFormatter = &log.TextFormatter{}
 
@@ -17,6 +32,9 @@ var (
 
 	// GitCommit hash, set at compile time
 	GitCommit = "unknown"
+
+	// Config is the global tool configuration
+	Config = GlobalConfig{}
 )
 
 // infoFormatter overrides the default format for Info() log events to
@@ -37,6 +55,22 @@ func version() {
 	os.Exit(0)
 }
 
+func readConfig() {
+	cfgPath := filepath.Join(os.Getenv("HOME"), ".moby", "linuxkit", "config.yml")
+	cfgBytes, err := ioutil.ReadFile(cfgPath)
+	if err != nil {
+		if os.IsNotExist(err) {
+			return
+		}
+		fmt.Printf("Failed to read %q\n", cfgPath)
+		os.Exit(1)
+	}
+	if err := yaml.Unmarshal(cfgBytes, &Config); err != nil {
+		fmt.Printf("Failed to parse %q\n", cfgPath)
+		os.Exit(1)
+	}
+}
+
 func main() {
 	flag.Usage = func() {
 		fmt.Printf("USAGE: %s [options] COMMAND\n\n", filepath.Base(os.Args[0]))
@@ -56,6 +90,8 @@ func main() {
 	flagQuiet := flag.Bool("q", false, "Quiet execution")
 	flagVerbose := flag.Bool("v", false, "Verbose execution")
 
+	readConfig()
+
 	// Set up logging
 	log.SetFormatter(new(infoFormatter))
 	log.SetLevel(log.InfoLevel)

src/cmd/linuxkit/pkg.go

@@ -3,6 +3,7 @@ package main
 import (
 	"fmt"
 	"os"
+	"os/exec"
 	"path/filepath"
 )
 
@@ -19,12 +20,37 @@ func pkgUsage() {
 	fmt.Printf("See '%s pkg [command] --help' for details.\n\n", invoked)
 }
 
+func setupContentTrust() {
+	// If it is already set there is nothing to do.
+	if _, ok := os.LookupEnv("DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE"); ok {
+		return
+	}
+	// If it is not set but it is needed this is checked at time
+	// of use, not all commands need it.
+	if Config.Pkg.ContentTrustCommand == "" {
+		return
+	}
+
+	// Run the command and set the output as the passphrase
+	cmd := exec.Command("/bin/sh", "-c", Config.Pkg.ContentTrustCommand)
+	cmd.Stdin = os.Stdin
+	cmd.Stderr = os.Stderr
+	v, err := cmd.Output()
+	if err != nil {
+		fmt.Printf("Failed to run ContentTrustCommand: %s\n", err)
+		os.Exit(1)
+	}
+	os.Setenv("DOCKER_CONTENT_TRUST_REPOSITORY_PASSPHRASE", string(v))
+}
+
 func pkg(args []string) {
 	if len(args) < 1 {
 		pkgUsage()
 		os.Exit(1)
 	}
 
+	setupContentTrust()
+
 	switch args[0] {
 	case "build":
 		pkgBuild(args[1:])